1. Field of the Invention
Subject matter of the invention is a method for the optical characterization of the internal structure and/or composition of a spatially extended, light scattering sample with the aid of an arrangement consisting of one or several light sources, one or several light detectors, and an apparatus suitable therefor.
2. Discussion of the Related Art
In the field of medical diagnostics, efforts have been made to make use of the internal structures of bodies to recognize diseases. This development began with the exposure of an object to light of a selected wavelength or white light and recording the intensity of the reflected light (Non-invasive methods for the quantification of skin functions; P. J. Frosch and A. M. Klingman (ed.), Springer Verlag Berlin/Heidelberg, 1993, pages 3 to 24 and 25 to 41). A drawback of this method is that it only allows the detection of surface properties as the information is determined by the absorptive or reflective properties of the surface. Another drawback of the method is the poor reproducibility in a quantitative evaluation.
Even scanning devices used to read in a page of text only scan the surface since the light, which is measured and reflected by light source and receiver essentially at the surface due to continuous exposure, must not or cannot penetrate the paper.
The basis for sonographic methods is to distinguish between different structures, e.g. different types of tissue, based on the different reflection factors for sonic waves; said reflection factors being proportional to the acoustic impedance (I. Krestel (ed.), Bildgebende Systeme fur die medizinische Diagnostik; Siemens AG, Berlin, Munchen, 2nd edition 1988, page 183 et seq.). Sonographic methods can be used to obtain information from the interior of a sample, but they are entirely dependent on the mechanical properties (density). This means that it is possible to differentiate and represent only structures with greatly varying acoustic impedances. When different soft tissue parts are analyzed, the differences in the acoustic impedance and/or the reflection factor are very small and show either none or only very weak contrasts.
Due to the optical properties of the tissue (scattering and absorption coefficient), the use of light with wavelengths in the near infra-red range (NIR) allows in particular in the field of in-vivo analytics the penetration of thicker tissue areas (some millimeters up to few centimeters) without damage (as is the case with X-rays). In currently known methods various theories are used where the photon path must be mathematically traced back to generate an image based on the measured light intensities; this is accomplished either by the use of measuring methods with a high time resolution (in the ps range) and/or with considerable calculation effort. The measuring apparatus used in these methods generally comprises a high performance, ultra-short pulsed laser, a complex optical unit for laterally scanning the sample surface, and an ultra-fast detector. Possible lasers are, e.g. mode-coupled gas ion laser (Ar, Kr) which synchronously pump dye lasers. Particularly suitable for the detection are streak cameras, microchannel photomultipliers, or Kerr-Shutter (L. Wang et al.: Ballistic Imaging of Biomedical Samples using ps optical Kerrgate; SPIE Vol. 1431, Time Resolved Spectroscopy and Imaging of Tissues (1991), page 97 ff.).
Both handling and costs restrict the use of these apparatus to highly qualified and highly specialized optical laboratories. Equivalent semi-conductor components which can be used for the same type of measurement and allow a necessary degree of miniaturization are presently not available and/or a future availability is not yet in sight. The instrumental difficulties are further complicated by the problem of image reconstruction. Since there are countless ways for the photons to travel from the light source to the detectors, it is not possible to give one unique reconstruction algorithm, as is possible in computer tomography, for example. Although first practical attempts at solving the problem have been made (S. R. Arridge et al.; New results for the development of infra-red absorption imaging; Proceedings of SPIE--The International Society of Optical Engineering, Vol. 1245, pages 92--103), a method for a successful reconstruction has not yet been shown. Moreover, owing to the computing times involved, these methods are not suitable for use in clinical diagnostics.
EP-A 0 387 793 describes a sensor that is placed on a skin section to generate an optical image of the tissue beneath it. However, this apparatus is not suitable for examining larger areas of the body as this would require a very large number of detectors. Moreover, the optical resolution of the generated image is not satisfactory.
DE-A-4341063 describes a method for determining density distributions, wherein the light passing through the tissue is evaluated with respect to the phase and amplitude of high-frequency modulated radiation.